Eclogitization and exhumation of Caledonian continental basement in Lofoten, North Norway Mark G. Steltenpohl1, Gabriel Kassos1, Arild Andresen2, Emma F. Rehnström3, and Willis E. Hames1 1Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA 2Department of Geosciences, University of Oslo, Box 1047, Oslo 3, Norway 3Geological Survey of Denmark and Greenland, Ø Voldgade 10, DK-1450 Copenhagen, Denmark ABSTRACT and Bergen Arcs allochthons that were later INTRODUCTION thrust onto Baltica during the main Scandian U-Pb and 40Ar/39Ar isotopic data com- (Siluro–Devonian) collision. Lofoten eclogites Eclogites are important indicators of plate bine with structural and petrological infor- appear to be ca. 50 m.y. older than the “late” tectonic processes but constitute only a very mation to allow insights into the timing of group of autochthonous, Scandian (ca. 425– minor volume of continental basement exposed Caledonian tectonic burial and exhumation 400 Ma) high-pressure and ultrahigh-pressure today at the Earth’s surface. The Western Gneiss of lower crustal rocks now exposed in the (UHP) eclogites of the Western Gneiss Region (WGR) of west Norway (Fig. 1) is argu- Lofoten Islands of North Norway (latitude Region (WGR), and the former preserve a ably the largest, best exposed, most accessible, 68° N). Severely retrogressed eclogites occur much longer (ca. 100 m.y.) exhumation his- and now best studied eclogitized continental in rare lenses or, even more rarely, hydrated tory. To date, there is no evidence to indicate basement terrane in the world (Dobrzhinetskaya shear zones within Archaean and Proterozoic either UHP or Scandian eclogite-facies meta- et al., 1995; Hacker et al., 2003; Hacker, 2007; granulite-facies Baltic basement gneisses. morphism in Baltic basement along coastal Walsh et al., 2007; Hacker et al., 2010). High- The timing of high-pressure metamorphism Norway north of latitude 64° N. Middle pressure (HP) and ultrahigh-pressure (UHP) in Lofoten has been diffi cult to determine Ordovician eclogites of Scandinavia are con- rocks of the WGR record deep-level subduction because retrogression has disturbed min- temporaneous with Taconic eclogites found in of the ancient Baltic margin beneath Laurentia eral isotopic systems and zircon apparently thrust nappes of the Appalachian Orogen. If during the Scandian (Siluro–Devonian) phase was not generated, leaving its tectonic sig- the Lofoten eclogites correlate to those in the of the Caledonian orogeny (Roberts and Gee, nifi cance uncertain. Recently discovered pre- Tromsø Nappe Complex (Uppermost Alloch- 1985; Andersen and Jamtveit, 1990). Following and post-kinematic felsic injections provide thon), then both terranes might represent UHP metamorphism, the WGR was educted out the opportunity to bracket the age of eclogiti- Laurentian relics emplaced during the Scan- or exhumed from underneath Laurentia along zation. U/Pb analyses of zircon and xenotime dian and left orphaned on the conjugate side the system of Early Devonian extensional faults from a prekinematic syenogranite dyke that of the orogen when the North Atlantic began associated with the Nordfjord-Sogn detachment cuts the mafi c host to one of the retro-eclogite to open in the Eocene. Lithologic, petrologic, (Norton, 1986; Seranne and Seguret, 1987; lenses intruded at 1800 ± 5 Ma, and we inter- kinematic, provenance, and palinspastic Andersen and Jamtveit, 1990; Osmundsen and pret the strong disturbance of its U-Pb sys- information favor, however, correlation with Andersen, 1994; Walsh et al., 2007). Coupled tem at 478 ± 41 Ma to approximate the age eclogites in transitional Baltic-Iapetan crust with the lack of a subsequent contractional oro- of eclogitization. Omphacite breakdown of the Seve Nappe Complex (Upper Alloch- genic overprint, such as that which typifi es most textures imply rapid isothermal decompres- thon), which provides a piercing point link- other mid-Paleozoic orogens (e.g., the Taconic, sion during initial uplift, followed by slow ing eclogites in autochthonous Baltic crust Acadian, and Alleghanian phases of the Appa- uplift. Syn-upper-amphibolite-facies thrust now exposed in the most internal parts of the lachian orogeny; see references in Miller et al. emplacement of the overlying Leknes Group orogen with those transported and preserved [2010]), the Caledonian lithospheric root was at ca. 464 Ma implies that the Lofoten base- in the Swedish foreland. We suggest a broad rather simply exhumed to emerge as the present- ment resided in the lower crust for ca. 14 m.y. twofold subdivision for the Eocaledonian day WGR. The pristine preservation in the WGR 40Ar/39Ar cooling dates from the retro-eclogites eclogite provinces into those that are of rocks and their structural geometries serve trace their exhumation path into the middle (1) Baltic derived, and (2) exotic with respect as a world-class example of deep-continental crust (hornblende ca. 433 Ma) where they to Baltica. Middle Ordovician eclogites falling subduction and very rapid (<10 m.y.) and large resided before being slowly elevated to levels under category (1) can further be sub divided magnitude (≥120 km throw) tectonic unroofi ng. at the base of the ductile-brittle transition into those occurring in autochthonous Baltic Scandinavian eclogite terranes are also found in the early Carboniferous (muscovite ca. basement (i.e., Lofoten), and those in thrust at a variety of tectonostratigraphic levels within 343 Ma). The Middle Ordovician Lofoten translated terranes. Apparently, continental the stack of Caledonian allochthons that were eclogites likely formed at a time similar to crust in a collisional setting can be subducted thrust eastwardly upon the Baltic basement. the “early” group of Eocaledonian eclogites to mantle depths and show only very sparse These older, Cambro–Ordovician eclogite ter- (ca. 505–450 Ma) found in the Tromsø, Seve, evidence of this tectonic history. ranes occur at various tectonostratigraphic levels Geosphere; February 2011; v. 7; no. 1; p. 202–218; doi: 10.1130/GES00573.1; 12 fi gures; 1 table; 1 supplemental table. 202 For permission to copy, contact [email protected] © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/1/202/3340348/202.pdf by guest on 25 September 2021 Eclogitization and exhumation of Caledonian continental basement in Lofoten, North Norway through targeted isotopic dating have we been Early Carboniferous Paleogeographic Reconstruction able to confi dently distinguish between which Highly allochthonous granite-on-granite shear zone is Proterozoic Post-Devonian rocks and which is Paleozoic and/or Caledonian Baltic cover (Hames and Andresen, 1996; Steltenpohl et al., 2003b, 2004, 2006). The paucity of Cale- Precambrian basement donian structures is thought to be the result of the limited availability of fl uids to facilitate ice e plastic deformation of the anhydrous, rigid e Figure 1. Carboniferous paleo- e e granulites (Bartley, 1982, 1984; Hodges et al., geographic reconstruction of the 1982; Steltenpohl et al., 2004). The eclogites in TNC Lofoten , likewise, are volumetrically miniscule, Norway-Greenland Caledonides, Lofotene depicting general distribution and are restricted in their occurrence to rare, e thin (meter scale) ductile shear zones (Wade, of eclogites (modified from area of SNC figure 2 e 1985; Kullerud, 1992, 1996; Markl and Bucher, Steltenpohl et al., 2006). Abbre- e viations: SNC—Seve Nappe 1997; Kassos, 2008). Eclogite shear zones in Complex, TNC—Tromsø Nappe e e continental basement rocks are exceedingly Complex. rare, and, as with those in Lofoten, are argued to have formed by eclogitizing fl uids that weak- e Western ened the rocks and promoted crystal-plastic Gneiss e e Baltic Shield fl ow and shear zone development (Austrheim, Region e Kilometers e 1987; Austrheim and Griffi n, 1985; Boundy et al., Bergen Arcs e 0 400 1992; Kullerud, 1992, 1996, 2000; Markl and den e Bucher, 1998; Markl et al., 1997, 1998a, 1998b; w Norway S Eclogite localities Steltenpohl et al., 2006). e Basement Herein we report fi eld, structural, and iso- e Thrust sheet topic results on a crystal-plastic shear zone on the Lofoten island of Flakstadøy (Figs. 1 and 2), which contains variably retrogressed lenses of eclogite, documenting it to be a Caledonian within the Caledonian nappe stack (Fig. 1). basement gneisses that project as the northern shear zone. During our mapping, we discov- Middle Ordovician eclogites in the Tromsø continuation of the WGR (Fig. 1), also con- ered felsic injections that both predate and Nappe Complex (Corfu et al., 2003) occur at tain eclogites, but they have not yet been dated postdate eclogitization, providing an opportu- the highest preserved structural levels, within (Wade, 1985; Kullerud, 1992, 1996; Markl and nity to attempt isotopically bracketing the age the exotic (Laurentian) Uppermost Allochthon Bucher, 1997; Steltenpohl et al., 2003b, 2006; of eclogi ti za tion. We also present U-Pb isotopic (Fig. 1). The eclogitic Seve Nappe Complex Kassos, 2008). As was earlier presumed for the dates on minerals from one of the felsic injec- (Fig. 1) constitutes the base of the Upper Alloch- WGR eclogites before more robust isotopic tions that combine with 40Ar/39Ar mineral cool- thon in northern Sweden, and is interpreted to methods were available to date them (Krogh, ing dates, and regional geological information, have been derived from the area of extended 1977), the Lofoten eclogites were